Semiconductor lasers are widely used in various wavelength bands, but recently the demand of green lasers of 530 nm-band is increased as micro projectors and high luminosity factor laser pointers. The semiconductor laser which outputs green is researched and developed in two modes that laser beam in 530 nm-band is directly oscillated and that infrared laser beam oscillated in 1060 nm-band is changed into laser beam of 530 nm-band by second harmonic generation (SHG). Especially in terms of the operational voltage and the wall-plug efficiency (WPE), the later mode using SHG is expected more advantageous.
The device using SHG requires as the master light source of 1060 nm-band a laser light source of the single longitudinal mode oscillation, and for example, a distributed feedback (DFB) laser+an SHG element, a distributed Bragg reflector (DBR) laser+an SHG element, etc. are proposed. In either of these structures, the oscillation wavelength of the semiconductor laser and the wavelength of the quasi phase matching (QPM) of the SHG element have to be precisely matched, and for the wavelength control, generally a thermoelectric cooler (TEC), such as a Peltier element, is used.
However, the power consumption of the TEC reaches about some watts, depending on used environmental temperatures, which has been a barrier to the improvement of the WPE. The modulation of the output intensity is made by modulating the output of the semiconductor laser, i.e., by modulating the drive current, which often drifts the oscillation wavelength from the QPM wavelength of the SHG element.
The structure of the SHG element combined with a master oscillator power amplifier (MOPA) structure including a DFB laser or a DBR laser, and a semiconductor optical amplifier (SOA) is proposed against the difference between the oscillation wavelength and the QPM wavelength. In the MOPA structure, the semiconductor laser is operated at a constant current, and the SOA modulates and adjusts the intensity, whereby the fluctuation of the oscillation wavelength of the semiconductor laser can be suppressed by suppressing the drift from the QPM wavelength of the SHG element.
The followings are examples of related: Japanese Laid-open Patent Publication No. 2000-228556; and Japanese Laid-open Patent Publication No. 2009-164443.
However, because of the very narrow wavelength spectrum of the QPM of the SHG element, influences cannot be disregarded even by a slight thermal crosstalk between the DFB laser and the SOA. For example, one kind of the pattern effect, that the oscillation wavelength of the semiconductor laser is drifted and a power change is caused by the drive signal of the SOA, has often caused waveform distortions in outputs of the SHG element.
Even in the MOPA structure, without the TEC, the difference between the oscillation wavelength of the semiconductor laser and the QPM wavelength of the SHG element is caused also by environmental temperature changes. This is caused by the difference in the temperature dependency between the QPM wavelength of the SHG element and the Bragg wavelength of the diffraction grating which decides the oscillation wavelength of the DFB laser or the DBR laser.
Furthermore, in the MOPA structure, the large difference in the temperature dependency between the oscillation wavelength of the semiconductor laser and the gain spectrum of the SOA is one problem. In the operation without the TEC to improve the WPE, it is preferable to design the detuning between the oscillation wavelength of the semiconductor laser and the gain peak wavelength at the higher temperature condition under which the output characteristics are largely degraded. However, because of the different temperature dependency between the oscillation wavelength of the semiconductor laser and the gain peak wavelength of the SOA as described above, the detuning becomes too large at lower temperature. Resultantly, Fabry-Perot modes often oscillate on the lower temperature, which, even in the MOPA structure, have been a problem in realizing the green laser module of low power consumption without the TEC.